Novel catalytic hydrogenolysis of silyl enol ethers by the use of acidic ruthenium dihydrogen complexes

Treatment of 1-trimethylsilyloxy-1-cyclohexene (1a) in the presence of a catalytic amount of the acidic dihydrogen complex [RuCl(η2-H2)(dppe)2]OTf (4a) [dppe=1,2-bis(diphenylphosphino)ethane, OTf=OSO2CF3] (10 mol.%) under 1 atm of H2 in anhydrous ClCD2CD2Cl at 50 °C for 8 h afforded cyclohexanone (3a) and Me3SiH in quantitative NMR yields. Silyl enol ethers such as 1-triethylsilyloxy-1-cyclohexene (1b), 1-t-butyldimethylsilyloxy-1-cyclohexene (1c), and other trimethylsilylethers (1d, 1e, and 1f) reacted similarly with H2 to afford the corresponding ketones and trialkylsilanes. The direct proton transfer from H2 to the trimethylsilyl enol ethers (1a and 1d–1f) was confirmed by the experiments employing D2 gas, where α-monodeuterated ketones (3a′ and 3d′–3f′) were obtained in high yields. The enantioselective protonation of prochiral silyl enol ethers with 1 atm of H2 by employing [RuCl(η2-H2)((S)-BINAP)2]OTf (4e) [BINAP=2,2′-bis(diphenylphosphino)-1,1′-binaphthyl] and [RuCl(η2-H2)((R, R)-CHIRAPHOS)2]OTf (4f) [CHIRAPHOS=2,3-bis(diphenylphosphino)butane] showed that no enantioselectivity was observed in either catalytic or stoichiometric protonation reactions under various reaction conditions. The reaction of [RuHCl(dppe)2] (5a) with one equivalent of Me3SiOTf under 1 atm of H2 produced rapidly 4a, concurrent with the formation of Me3SiH. Based on these studies, the mechanism for this novel hydrogenolysis of silyl enol ethers is proposed which involves heterolytic cleavage of the coordinated H2 on the ruthenium atom caused by the nucleophilic attack of the oxygen atom of enol ethers to give ketones and Me3SiOTf, and the subsequent reaction of the resultant complex 5a with Me3SiOTf under 1 atm of H2 to regenerate the original dihydrogen complex 4a. On the other hand, the stoichiometric reaction of a lithium enolate 6e with one equivalent of 4e at −78 °C in CH2Cl2 under 1 atm of H2 afforded 2-methyl-1-tetralone (3e) with 75% ee (S) in >95% yield, together with the formation of [RuHCl((S)-BINAP)2] (5e).